Optical measurement systems are employed in a variety of industries, such as the semiconductor processing industry, for real-time monitoring of wafer modification and process control. Optical measurement systems may be integrated with a semiconductor processing tool and may be utilized in-situ for real-time process control or in-line for run-to-run feedback control. Typically, monitored processes include semiconductor etching, deposition, implantation and chemical mechanical planararization processes for film thickness and plasma monitoring applications.
Especially in the semiconductor processing industry, the use of increasingly variable material layers and features sizes (thinner/thicker layers, high aspect ratio features, very small features, mixed size features, highly variable reflectivity/absorption materials, and high layer count stacks) has led to difficulties in achieving necessary levels of measurement accuracy and precision. In addition to the increasing complexity of the semiconductors themselves, highly integrated single chamber multiple step processes and dynamic processing tool changes of mechanical parameters (e.g., aperture and working distances) cause variation in optical signal levels adversely affecting measurement accuracy and precision.
FIG. 1 shows a pictorial schematic of a typical prior art optical measurement system 100. Optical measurement system 100 includes light analyzing device 110, light source 120, optical assembly 130, optical fiber assembly 140, computer 150 and wafer 160. Light analyzing device 110 is commonly a spectrograph, spectrometer, monochromator or other light analyzing device providing wavelength discrimination. Light source 120 is either a continuous broadband emission source (e.g., tungsten halogen lamp or deuterium lamp) or a pulsed broadband emission source such as a xenon flashlamp. Optionally, narrowband continuous or pulsed emission sources such as lasers and/or light emitting diodes are used. Optical assembly 130 is designed to direct light of one or more wavelengths emitted from light source 120 onto wafer 160 which is typically a silicon semiconductor wafer, sapphire substrate or other workpiece. Optical assembly 130 commonly acts to either focus or collimate light from light source 120 onto wafer 160. Optical fiber assembly 140 is commonly a bifurcated optical fiber assembly which directs light from light source 120 to wafer 160 via optical assembly 130 and subsequently directs light collected upon reflection from wafer 160 via optical assembly 130 to light analyzing device 110. Computer 150 is used to control light analyzing device 110 and light source 120 and is also used to analyze data collected by light analyzing device 110. Computer 150 may also provide signals to control external systems such as semiconductor processing tools (not shown).
Reflectometry in the form of interferometric endpointing is widely used in the semiconductor industry for monitoring the state of a wafer process within a semiconductor processing tool by using optical signals reflected from a wafer being modified within the processing tool. While interferometric endpointing techniques may vary with the particular application and process, typically the light emission intensities are monitored at one or more predetermined wavelengths. Depending on the process, various algorithms may be employed for deriving trend parameters, often related to thicknesses of various layers or features of the wafer, from the light intensities that are useful in assessing the state of the semiconductor process and the in-process wafer, detecting faults associated with the process, processing tool or other equipment. Although commonly named “endpointing” and historically implying the detection of the end of a process; interferometric endpointing has evolved to include monitoring and measurement during all times of a process cycle.
With specific regard to monitoring and evaluating the state of a wafer within a processing tool, FIG. 2 illustrates a typical prior art process 200 for employing interferometric endpointing to monitor and/or control the state of a workpiece within a plasma processing tool. The present method is greatly simplified for expedience. Details of certain processes and implementations are provided by review of US Patent Application Number 20130016343, included herein by reference. Process 200 typically begins by directing light onto the workpiece of interest (step 210). Light directed onto the workpiece is then reflected from that workpiece (step 220) and subsequently detected (step 230). Detection is commonly associated with conversion to electrical signals, the signals are typically amplified and then digitized and passed to a signal processor for analysis (step 240). The signal processor employs one of more algorithms that is/are specific to the particular production process and the characteristics of the workpiece being monitored. The selection of the proper algorithm, as well as parameter values, for the particular process is imperative to achieving a valid result. Without being too specific, the algorithm analyzes intensity signals and determines trend parameters that relate to the state of the process and can be used to access that state, for instance, end point detection, etch depth, film thickness, faults, plasma instability, etc. (step 250). The results are output (step 260) for use by external control systems and/or engineers and then used for monitoring and/or modifying the production process occurring within the plasma processing tool (step 270).